Control valves and other devices which are operated by such actuators control the flow of control fluids, at high pressures, or production fluids such as liquid or gaseous hydrocarbons. It is particularly important that in the event of failure of the prime mover of the actuator, such as for example its power supply, the actuator should remain in or revert to a predetermined mode in which (usually) the controlled device such as a control valve is closed. The invention therefore concerns a reliable and comparatively simple actuator of this kind.